A polyolefin-based microporous film has been widely used as a battery separator, a separator filter, and a membrane for microfiltration, due to its chemical stability and excellent physical properties. Meanwhile, for the battery separator, the microporous structure is required to have a spatial separation function between positive and negative electrodes and a microporous structure for high ionic conductivity. Recently, it has been further required to enhance the characteristic of the separator for the thermal stability and electrical stability upon charging and discharging of the secondary battery, according to the tendency of the secondary battery toward the high-capacity and high-power. In case of the lithium secondary battery, if the thermal stability of the separator is deteriorated, the separator may be damaged or deformed by increase of temperature in the battery and thus an electrical short may occur between the electrodes. Therefore, there is a risk that the battery may be overheated or ignited. The thermal stability of the battery is affected by shutdown temperature, meltdown temperature, high temperature melt shrinkage and the like.
The separator having the excellent thermal stability at high temperature is prevented from being damaged at the high temperature, thereby preventing the electrical short between the electrodes. If the electrical short occurs between the electrodes due to dendrite generated during charging and discharging processes of the battery, the battery generates heat. At this time, in case of the separator having the excellent thermal stability at high temperature, it is prevented that the separator is damaged and thus the battery is ignited or exploded.
In order to increase the thermal stability of the separator, there have been proposed a method that crosslinks the separator, a method that adds inorganic matterparticles, and a method that mixes a heat-resistant resin with a polyolefin resin or forms a coating layer.
The crosslinking method of the separator is disclosed in U.S. Pat. Nos. 6,127,438 and 6,562,519. In these methods, a film is treated by electron beam crosslinking or chemical crosslinking. However, in case of the electron beam crosslinking, there are some disadvantages such as necessity for an electron beam crosslinking apparatus, a limitation of production speed, a variation in quality according to non-uniform crosslinking. And in case of the chemical crosslinking, there are also some disadvantages in that it has complicated extruding and mixing processes, and a gel may be generated at a film due to the non-uniform crosslinking.
Meanwhile, in U.S. Pat. No. 6,949,315, there is disclosed a method of enhancing the thermal stability of the separator by mixing an ultra high molecular weight polyethylene with inorganic particles like titanium oxide of 5 to 15 weight %. However, in this method, there are some disadvantages such as increase of extruding load, deterioration of the extruding and melt-kneading ability, and occurrence of incomplete stretching due to using of the ultra high molecular weight polyethylene, as well as inferiority in the mixing, variation in quality and generation of pinholes due to using of inorganic particles. Further, physical properties of the film are also deteriorated due to lack of interface compatibility between the inorganic particles and the high molecular resin.
In U.S. Pat. No. 5,641,565, there is disclosed a method in which an excellent heat-resistant resin is melt-kneaded. In this method, an ultra high molecular weight resin having an average molecular weight of 1,000,000 or more is needed to prevent deterioration of the physical properties due to adding of polyethylene, polypropylene and inorganic particles. Further, since an additional is also needed to separate and remove the used inorganic materials particles, the manufacturing process is very complicated.
In Japanese Patent Publication No. 2004-161899, there is disclosed a microporous film which contains polyethylene and non-polyethylene thermoplastic resin having excellent heat-resistance and being not completely melted but minutely dispersed when being mixing with the polyethylene. However, there is a disadvantage that the microporous film manufactured by this method has a non-uniform thickness due to particulate heat-resistant resin. If the microporous film has the non-uniform thickness, the defective proportion in assembling of battery is increased and thus the productivity is reduced. Also, after the assembling of battery, an electrical short occurs, thereby deteriorating safety.
In U.S. Pat. No. 5,691,077 and Japanese Patent Publication No. 2002-321323, there are disclosed methods of forming additionally heat resistant layer on a polyolefin-based microporous film. In these methods, a polypropylene layer is provided by a dry or wet process, but a heat-resistant layer is stretched and it is difficult to basically prevent heat shrinkage due to limitation of a melting point of polypropylene. Therefore, there is limitation in manufacturing of a high heat-resistant separator. Further, in Korean Patent Publication No. 2007-0080245 and International Publication No. WO2005/049318, polyvinyldene fluoride copolymer that is a heat-resistant resin is used as a coating layer so as to enhance the heat-resistance of the separator and the thermal stability of the battery. However, since the resin is easily dissolved or gelled in an organic solvent such as propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate, which is used as a non-aqueous electrolyte of a battery, there is limitation in enhancing the thermal stability of the battery.
In Japanese Patent Publication No. 2002-355938, there is disclosed a microporous polyolefin composite film in which a high heat-resistant resin is used. In this film, the high heat-resistant resin is applied to a polyolefin microporous film by the phase separation. However, it is difficult to provide efficient permeability by a pore forming method in which a single resin is phase-separated by a dry process when forming a coating layer of the film. Further, since phase separation size and uniformity are considerably changed according to the drying conditions such as humidity, temperature and so on, there is limitation in manufacturing the separator having uniform quality.
With respect to the heat-resistance as one of the main characteristics of the battery separator, the conventional methods have a limitation in the heat-resistance of the resin itself, or the applying of the heat-resistant resin does not contribute to the improvement of the heat-resistance of the separator. And other physical properties like gas permeability are low or do not mentioned, and also the quality uniformity is poor. Further, when the separator manufacture by the conventional methods are actually applied to the battery, there is a problem that it is not provide constant thermal stability under the conditions such as high temperature, high voltage and organic electrolytes.